doc: include list of extensions

doc/qubes-ext.rst

@@ -1,8 +1,22 @@
 :py:mod:`qubes.ext` -- Qubes extensions
 =======================================
 
+Module contents
+---------------
+
 .. automodule:: qubes.ext
    :members:
    :show-inheritance:
 
+Extensions defined here
+-----------------------
+
+.. autoclass:: qubes.ext.admin.AdminExtension
+.. autoclass:: qubes.ext.block.BlockDeviceExtension
+.. autoclass:: qubes.ext.core_features.CoreFeatures
+.. autoclass:: qubes.ext.gui.GUI
+.. autoclass:: qubes.ext.pci.PCIDeviceExtension
+.. autoclass:: qubes.ext.r3compatibility.R3Compatibility
+.. autoclass:: qubes.ext.services.ServicesExtension
+
 .. vim: ts=3 sw=3 et